Monitoring pressures and pressure changes in a human body is often an important component of a medical or surgical diagnosis or therapy. For example, pressure changes in various body chambers, such as blood pressures in chambers of the heart, may be used for diagnosis and/or treatment of a number of conditions. One or more pressure sensors positioned in a heart chamber, for example, may allow a physician to monitor the functional ability of the heart to pump blood, such as in a patient suffering from congestive heart failure. Blood pressure monitoring in the heart may also be used to automatically activate or adjust a pacemaker, such as a rate-responsive or pressure-responsive pace maker. In some cases, one or more pressure sensors may be implanted in a heart to sense chamber pressures over an extended time period and adjust pacemaker timing or the like. Both rate-responsive pacemakers and techniques for measuring intracardiac pressures are known in the art.
Other bodily pressures and pressure changes may also be used in medical and surgical diagnosis and treatment. Pressure changes across various valves or sphincters, within body chambers or tracts such as the digestive tract, bladder filling and voiding pressures, and the like may be sensed and measured for use in a medical or surgical context.
An ideal medical pressure sensor would be both very sensitive and very stable (i.e., having very limited drift over time), while also being relatively small. Some medical pressure sensing devices, for example, should be small enough to be conveniently implanted at a desired site in a patient or to be carried on a catheter.
Advances in micromachined sensor technology have been made in order to develop small pressure sensing devices. Micromachined sensors typically measure an environmental variable, such as a pressure or acceleration, by detecting the strain induced on a sensor element, i.e., transducer. The sensor converts the strain into an electrical signal by measuring the resistance of the strained element, such as is done in piezoresistive-based sensors, or the change in vibrational frequency of that element, such as is done in resonance-based sensors. Specifically, pressure sensors detect the strain in a diaphragm that is distended in response to a pressure change, while accelerometers measure the strain caused by the displacement of a proof mass under an inertial load.
Piezoresistive pressure sensors make up the bulk of commercially available microfabricated pressure sensors. In general, this type of sensor uses two piezoresistors positioned on a circular or rectangular diaphragm to form a 90 degree angle. FIGS. 1 and 1B, for example, show a prior art microfabricated pressure sensor 10 having a circular diaphragm 12 with a radially oriented piezoresistor 16 and a circumferentially oriented piezoresistor 14. The two resistors 14, 16 are connected at one point to an output 17 of the sensor 10. The other two ends of the serially-connected resistors 14, 16 are connected to either voltage 13 or ground 15. When the trans-membrane pressure of such a diaphragm increases, the resistance of one of the resistors increases, and the other decreases. The effectiveness of the chip is adversely effected, however, by the fact that one resistance also increases and the other decreases when force is applied to the chip as a whole, such as bending, stretching and twisting forces. The sensitivity to such forces on the chip is inversely related to chip dimensions, so that the smaller the chip, the more sensitive it is to forces exerted on the chip. Such chips may be referred to as “single-point” sensors, in that they sense forces at essentially one location on a diaphragm.
In an improvement over single-point sensors, some currently available sensors include two resistors located along the perimeter of a diaphragm at separate locations, as shown in FIG. 1A. In this pressure sensor 10a, the radially oriented piezoresistor 16a and the circumferentially oriented piezoresistor 14a are distanced approximately ninety degrees apart along the perimeter of the diaphragm 12a. Thus, sensor 10a may have reduced sensitivity to stretching and bending, since the piezoresistors 14a and 16a cancel each other out somewhat. However, such a sensor 10a is equally sensitive to twisting forces as the sensor 10 shown in FIG. 1, because twisting is sensed by the piezoresistors 14a, 16a as pressure against the diaphragm 12a. 
Over extended periods of use, currently available pressure sensors experience drift. Drift is the distorting changes to base line readings which occurs as a result of a number of ambient factors. Drift normally occurs over time in pressure sensors. The variable quality of baseline sensor data drift in the sense of output interferes with obtaining data which accurately reflects changes in physiologic parameters. Drift obscures accurate data both by producing false positive and false negative readings. By example, false negative results can occur when drift of base-line data readings distorts or fully obscures physiologic parameter changes in signal which would otherwise be indicative of a disease state. This occurs when the drift brings a “0” base line level into a negative range. Conversely, when sensor drift is in a positive range it can be mistaken for a change in biological parameters, running the risk of a false indication of a disease state. Unfortunately, drift is typically unpredictable, and so can not be simply factored out of calculations in order to compensate for these data distortion.
It is a requirement for implantable pressure sensors that they have very stable output. This quality is necessary to assure that the data readings from the sensors are a true reflection of the pressure that they are designed to measure. The drift characteristic of many pressure sensors can be problematic with implanted sensors, where recalibration opportunities are limited or impractical. Because of the limited ability to recalibrate implanted sensors, the failure of currently available pressures sensors to remain stable (i.e., free of drift) in base-line data output has made them unsuitable for long term implantable use.
It would be an important advancement in the art if a micromachined pressure sensor were available that was resistant to drift in order to make the many advantages of micromachined sensors available for long term implantation applications by researchers and clinicians.
Relevant Literature. Methods for pressure-modulated rate-responsive cardiac pacing are described in U.S. Pat. No. 6,580,946. Techniques for monitoring intra-cardiac pressures are described in U.S. Pat. Nos. 5,810,735, 5,626,623, 5,535,752, 5,368,040, 5,282,839, 5,226,413, 5,158,078, 5,145,170 and 4,003,379.